FIG. 4 shows a conventional semiconductor integrated circuit. In FIG. 4, reference numeral Q1 designates a field effect transistor (referred to as FETQ1 hereinafter) and reference numerals T1 and T2 designate microwave lines connected to a source and a drain of the FETQ1, respectively. Reference numerals C5 and C6 designate capacitors connected to the microwave lines T1 and T2, respectively. Reference numeral R1 designates a resistor connected to a gate of the FETQ1, reference numeral C8 designates a capacitor connected to the resistor R1, reference numeral T3 designates a 1/4 wavelength line connected to the microwave line T1 and the capacitor C5, and reference numeral C7 designates a capacitor connected to the microwave line T3.
Then, operation thereof will be described. A drive signal input terminal S3 connected to a the connection of the resistor R1 and the capacitor C8 is used to control switching of the FETQ1. In addition, the 1/4 wavelength line T3 and the capacitor C7 apply a source voltage bias of the FETQ1, supplied from a power supply terminal V3 connected to the connection of the 1/4 wavelength line T3 and the capacitor C7. A high frequency signal is input from Rin and output to Rout. When the drive signal input terminal S3 becomes high, the FETQ1 is turned ON and the high frequency signal input from Rin is output to Rout. When the drive signal input terminal S3 becomes low, the FETQ1 is turned OFF and the high frequency signal input from Rin is not output to Rout.
The resistor R1 is generally set sufficiently higher than the impedance of the microwave lines T1 and T2. Therefore, when the FETQ1 is ON, the high frequency signal is prevented from leaking to the gate side of the FETQ1 through the capacitance Cgs between the gate and source of the FETQ1. In addition, when the mutual conductance Gm of the FETQ1 is fairly high, oscillation is prevented. Furthermore, electrostatic destruction of the gate of the FETQ1 is prevented.
The capacitor C8 and the resistor R1 are a RC low-pass filter circuit the capacitance of the capacitor C8 is set at a large value so as to be sufficiently low impedance to the high frequency signal so that the high frequency signal does not leak from the input terminal S3. In addition, although the resistor R1 is used in the example shown in FIG. 4, a 1/4 wavelength line is sometimes used instead of the resistor R1.
Since the conventional semiconductor integrated circuit is structured as described above, a relatively large drive circuit such as a TTL circuit is required in order to control the drive signal input terminal S3, to drive the capacitor C8 and the gate of the FETQ1, with a result that power consumption is increased and a switching speed of the FETQ1 is slow because it is necessary to charge and discharge the capacitor C8.